The pelvic region of an individual generally consists of the left and right ilium and the sacrum. The sacrum articulates with both the left and right ilium via the left and right sacroiliac joints. Both the left and right ilium are connected mutually to each other through the pubic symphysis or SI joint by a sacroiliac joint cartilage. This entire complex may be referred to as the "pelvic ring."10 The sacroiliac joint cartilage is different from cartilage found in other parts of the body. The structural development of the cartilage in the sacroiliac joint is not the same as in the other joints of the body. First of all, the normal smooth hyaline cartilage is absent. From the end of puberty on, the cartilaginous covering of the sacrum is relatively soft and thick. Microscopically, the sacroiliac cartilage, especially of the ilium, develops a rougher tissue than cartilage in other synovial joints. Secondly, macroscopic ridges develop on the sacroiliac joint cartilage surfaces along with complementary grooves on the opposite joint surfaces. This combination of microscopic roughness as well as macroscopic ridges and grooves provides the SI joint with more springing stability than motion.
In the frontal plane of the pelvic ring, the SI joint surfaces converge caudally. This converging position of both joint surfaces is beneficial in supporting body weight. On the other hand, the sacrum has a wedge form and is driven by the load of the spinal column between the two innominate bones, which in effect pushes the bones outwardly from each other.
The lumbar spine, pelvic ring and hip joints of an individual form a functional unit. Almost every movement of the lumbar spine has an influence on the pubic symphysis and both SI joints. The movements of an individual generate forces which are transferred through the sacrum and SI joints which are communicated or transferred to the hip joints and then to the remaining parts of an individual's lower extremities. In some individuals, movement may cause the load of the spinal column to push the two innominate bones outwardly, and when the ligaments are intact this will have a self-bracing stabilizing effect. If, however, the ligaments are lax, the self-bracing system will be less effective and then instability will occur.
Women, following childbirth, and other individuals may suffer from instability of the sacrum-ilium joint or (SI joint) or pubic symphysis. The instability of the SI joint or pubic symphysis may be treated over a period of time through muscle strengthening activities usually associated with a daily exercise program such as walking. In general, an individual through exercise attempts to strengthen the gluteus maximus muscle which is individually attached to the sacrum as well as to the ilium. In addition, a stabilizer belt for the SI joints and/or the pubic symphysis is beneficial for reduction of instability of the sacrum-ilium joint. No such belt is known which prevents the innominate bones from outward separation from each other, which in turn facilitates the alignment of the grooves and ridges of the sacrum-ilium joint surfaces for firm engagement to each other. The enhanced engagement of the grooves and ridges significantly reduces the rehabilitative time required to cure instability of the sacrum-ilium joint.